Rackspace was never the most affordable; their differentiator was service. Digital Ocean had very little to do with their decline.
The biggest factor was the shift in corporate mindset from “we need to operate our own servers” to “we need to orchestrate services running on someone else’s servers.” Probably AWS is most to blame for that.
Rackspace tried to make the jump across that chasm several times but they were trapped on the wrong side of the Innovators Dilemma. All their existing customers expected (expensive) full phone support. And new customers who wanted automated cloud systems with minimal support did not look to Rackspace for that.
Eventually they leaned entirely toward service and started reselling and supporting all the major cloud platforms. I would bet that is the majority of their revenue today. Any hosting they do should be considered a legacy business.